The fields of application aimed at in the present invention are those similar to asbestos, namely, where the product required should have a very low heat conductivity as well as excellent resistance and dimensional stability at high temperatures, namely, at temperatures higher than 1000.degree. C. By way of indication, these fields cover, for example, fire fighting clothing, fire trap partitions for flame temperatures exceeding 1000.degree. C., heat insulation for thermal treatment furnaces used in metallurgy, the siderurgy, the ceramic industry or in cassting molten metals.
Until the discover of the carcinogenic effects of asbestos, asbestos structures such as thermal suits, gloves, mats and the like as thermal and flame shields, have been preferred in industry. Asbestos possesses the preferred qualities of light weight flexible, substantially no flame shrinkage, and the ability to form fabrics. Asbestos had also been a preferred fiber for forming molten metal shields because the molten metal did not adhere and the structure absorbed impact by particles so that a ricochet problem was not present. Also, asbestos exhibited good shape reforming characteristics.
The use of various synthetic fibers such as Kevlar (Trademark), Teflonic (Trademark), polybenzylimidazoles, and the like, have been used as replacements but have been proved to be unsatisfactory and costly. Use of such prior art synthetic fabrics provded to be abrasive, irritating to the skin, too bulky and heavy when a suitable thickness was obtained for thermal insulation. Also, there was some shrinkage under a direct flame which could be critical if the thickness is critical. In addition, none of the fibers formed a structure which could absorb impact of particles so as to avoid substantial ricochet of particles.
Ceramic fibers have also been employed in fire fighting structures either alone or in combination with synthetic fibers. However, ceramic fibers have high bulk densities so that they cannot be used in many practical situations and they provide limited mobility when made into garments.
U.S. Patent application Ser. No. 856,305, filed October 10, 1987, of McCullough, et al, entitled "Carbonaceous Fibers with Spring-Like Reversible Deflection and Method of Manufacture", which is herewith incorporated by reference, discloses non-linear carbonaceous fibers which can be utilized in the invention. The carbonaceous fibers have a carbon content at least 65% and are prepared by the heat treatment of stabilized polymeric fibers, particularly stabilized acrylic fibers. Such fibers derived from acrylic fibers have a nitrogen content of 5 to 35%. Preferred precursor fibers are derived from pitch (petroleum or coal tar), polyacetylene, polyacrylonitrile based materials, polyphenylene, and the like.
U.S. Pat. No. 4,359,783 to Andrews, which is herein incorporated by reference, discloses garments comprised of synthetic fibers which can be utilized in hot or cold environments. However, a higher bulk density of the synthetic fibers is necessary to achieve the same thermal protection capable with the carbonaceous fibers used in the present invention.
The carbonaceous fibers used in the invention according to the test method of ASTM D 2863-77 have a LOI value greater than 40. The test method is also known as "oxygen index" or "limited oxygen index" (LOI). With this procedure the concentration of oxygen in O.sub.2 /N.sub.2 mixtures is determined at which the vertically mounted specimen-ignited at its upper end-just continues to burn.
The term "stabilized" herein applied to fibers or tows which have been oxidized at a specific temperature, typically less than about 250.degree. C. for PAN fibers, provided it is understood that in some instances the filament and or fibers are oxidized by chemical oxidants at lower temperatures.
The term "Reversible Deflection" or "Working Deflection" is used herein as it applies to a heical or sinusoidal compression spring. Particular reference is made to the publication "Mechanical Design-Theory and Practice", MacMillan Publ. Co., 1975, pp 719 to 748; particularly Section 14-2, pages 721-24.